The present invention relates generally to electric powered trip units, such as circuit breakers and more particularly to signal conditioning and data acquisition circuitry used to collect data representative of conditions in the circuit breaker.
In a typical factory-power distribution system, power is generated by a power generation company and supplied to a factory and thereafter distributed around the factory to various equipment such as, for example, motors, welding machinery, computers, heaters, lighting, and the like.
Power distribution systems of this type are typically centrally located in switch gear rooms or substations. From there, power is divided up into branches such that each branch supplies power to a portion of the factory and/or specified loads. Frequently, transformers are disposed throughout the factory to step down the supply voltage to that required by specific pieces of equipment or portions of the factory. Therefore, a factory-power distribution system typically has a number of transformers servicing various types of equipment in various areas. Inherent with this, is the high cost of the power-distribution equipment such as transformers, as well as the cost of the equipment to which power is being supplied. Therefore, it is quite common to provide protective devices such as circuit breakers or fuses in at least each branch so that not only may each piece of equipment be protected but any problems associated with one piece of equipment does not ripple to adjacent or interconnected pieces of equipment. Further, providing fuses or circuit breakers in each branch can help minimize down time since specific loads may be energized or de-energized without affecting other loads thereby creating increased efficiencies, lower operating and manufacturing costs and the like.
Typically, when circuit breakers are utilized, they are used to detect more than just large overcurrent conditions caused by short circuit faults. In addition, they frequently detect lower level long-time overcurrent conditions and excessive ground currents. The simplest form of circuit breakers are thermally tripped as a result of heating caused by overcurrent conditions and, in this regard, are basically mechanical in nature. These mechanical-type breakers are incorporated into almost all circuit breakers, regardless of whether or not additional advanced circuitry is provided since they are extremely reliable over a long life cycle and provide a default trip-type level of protection.
Some types of circuit breakers utilize electronic circuitry to monitor the level of current passing through the branch circuits and to trip the breaker when the current exceeds a pre-defined maximum value. Electronic circuit breakers are adjustable so as to fit a particular load or condition by the end user without designing or specifying different breakers. Breakers of this type typically include a microcontroller coupled to one or more current sensors. The microcontroller continuously monitors the digitized current values using a curve which defines permissible time frames in which both low-level and high-level overcurrent conditions may exist. If an overcurrent condition is maintained for longer than its permissible time frame, the breaker is tripped.
Microcontrolled breakers may also include the ability to calculate root mean square (RMS) current values. This is necessary in order to prevent erroneously tripping a circuit breaker when a non-linear load, such as a welding machine, is coupled to the branch that it is protecting. The reason for this is that non-linear loads tend to produce harmonics in the current waveform. These harmonics tend to distort the current waveform, causing it to exhibit peak values which are augmented at the harmonic frequencies. When the microcontroller, which assumes that the current waveform is a sinusoidal current waveform, detects these peaks it may therefore trip the breaker even though the heating effect of the distorted waveform may not require that the circuit be broken.
Further, microcontrollers in some circuit breakers are used to monitor and control or account for other types of faults, such as over or under voltage conditions and phase loss or imbalances. Such microcontrollers operate solenoids which are operatively connected to the trip mechanism of the circuit breaker. Therefore, while the thermal overload (mechanical) portion of the breaker will operate the trip mechanism, the solenoid will operate at the instruction of the microcontroller (or sometimes also at the instruction of external signals) to allow the trip mechanism to trip the associated circuit breaker.
Further, as a result of the flexibility and breadth of protection that microcontrollers can provide when used in conjunction with circuit breakers, their use in circuit breakers is becoming more and more prevalent to the point of being standard. However, this presents another problem in that microcontrollers and the associated circuitry require power. Such power may be typically provided in one of three ways or a combination thereof: batteries, externally-supplied power, or power provided by potential transformers. Most circuit breakers include one power supply having a battery back-up for supplying all of the controllers for the entire substation or switch gear closet.
Moreover, the monitoring of power characteristics is being demanded more and more frequently in load control equipment and particularly in Molded Case Circuit Breakers (MCCB) as is frequently found in use in industry. Such power components include, RMS and peak voltage, current and power, either by phase or in total, and power factor related components. For examples, utilities and industrial customers are increasingly interested in performing end-use load studies. These studies typically include collecting interval power data so as to monitor and control energy consumption. While this is often done at a main load center, due to the increased costs and problems associated with time of use power consumption, such monitoring is being done closer to the individual end-use loads (i.e., motors, etc.). In this fashion, industrial customers are given a financial incentive to curtail power consumption when the cost of power is high as well as being able to more carefully and cost-effectively manage their power consumption by knowing where in their plant significant amounts of energy are being used.
During power monitoring, a discrete energy transducer is installed on the equipment or circuit to be monitored. This transducer generates a digital pulse output via a mechanical or solid state relay with the frequency of the pulse output being proportional to the magnitude of the measured quantity. This digital pulse output is either hard wired or communicated via a power line-carrier system to a discrete pulse data recording device where it is time stamped.
Because it is desired to do monitoring of currents using discrete energy transducers, such as current transformer (CTs), it is desirable to receive a clean or noiseless signal in the data acquisition devices. Thus, there is a need for electronic circuitry which allows for a substantially clean signal to be communicated to data acquisition hardware. Further, there is a need for an electronic circuit which provides for a clean signal to be provided to measurement devices.
One embodiment of the invention relates to a data acquisition system for a circuit breaker. The data acquisition system includes a signal source providing a source signal, a microprocessor, and a signal conditioning circuit coupled to the microprocessor and configured to receive a periodic signal from the microprocessor. The data acquisition system also includes a chopping circuit coupled to the signal conditioning circuit and coupled to the signal source, the configured to chop the source signal in response to the periodic signal.
Another embodiment of the invention relates to a chopping circuit coupled to a signal source, the signal source providing a source signal. The chopping circuit includes a signal conditioning circuit receiving a periodic signal, a measurement branch coupled to the signal source, and a switching branch coupled to the signal conditioning circuit and coupled to the signal source. The switching branch is switched in response to the periodic signal and measurement, across the measurement branch, is carried out in response to the periodic signal.
Yet another embodiment of the invention relates to a circuit breaker. The circuit breaker includes a transducer providing a source signal representative of current flowing through the circuit breaker and an electronic trip unit including a data acquisition circuit having a sampling frequency. The data acquisition circuit includes a microprocessor having a clock frequency, a signal conditioning circuit coupled to the microprocessor and receiving a periodic signal from the microprocessor, and a chopping means coupled to the signal conditioning circuit and coupled to the current transducer. The chopping means chops the source signal in response to the periodic signal, to provide a measurement signal.